1. Field of the Invention
The present invention relates to a multiplex control system including a main controller and a subsidiary controller with different control characteristics, the system being capable of getting rid of the output error between these controllers.
2. Description of the Prior Art
Many of conventional multiplex control systems have been organized by combining a plurality of controllers made of the same hardware means and software means. FIG. 1 shows, as an example of such multiplex control systems, the arrangement of a dual control system. The system includes a process data input unit 10 receiving process data, e.g., pressure, temperature, flow rate, etc., from process lines (not shown), a main controller 11, a subsidiary controller 12, a data link unit 13, a switch unit 14, and a control object 15, e.g., valves. The operation of this system is as follows. In FIG. 1, process data collected on process lines (not shown) are received by the process data input unit 10 and delivered to the main controller 11 and subsidiary controller 12. The main controller 11 performs computation based on the data and transmits the result through the data link 13 to the subsidiary controller 12. In the event of a failure of the main controller 11, a switch unit 14 operates to transfer control of the control object from the main controller 11 to the subsidiary controller 12.
In this case, computational data processed by the main controller 11 is transmitted uninterruptedly to the subsidiary controller 12 through the data link unit 13, and therefore the control operation for the control object 15 is switched from the main controller to the subsidiary controller smoothly in a bumpless fashion. However, such a conventional multiplex control system, when applied to a system including a main and subsidiary controllers of different hardware structures, e.g., analog and digital devices, encounters difficulty in tracking computational data at the occurrence of a failure, and furthermore there are technical problems of realizing the bumpless switching of controllers.